Inherent limitation in the expressability of XML markup?

Hi Folks,

XPath is a non-XML syntax.

XSLT, Schematron, and XML Schema 1.1 make heavy usage of XPath. The role of XPath seems to expand with each new version of these technologies.

Why?

Example: consider this XML snippet:

<meeting>
    <start-time>08:00:00</start-time>
    <end-time>09:00:00</end-time>
</meeting>

In both Schematron and XML Schema 1.1 the co-constraint between the meeting's start time and end time is expressed using XPath:

  meeting/start-time lt meeting/end-time

Why isn't this co-constrain expressed using XML markup? Why do we resort to non-XML syntax?

Does it indicate an inherent limitation in the expressability of XML markup? Has anyone characterized this limitation?

/Roger